A gas turbine engine operates by drawing in ambient air, combusting that air with a fuel, and then forcing the exhaust from the combustion process out of the engine. A compressor, with a plurality of airfoils, rotates to draw in and compress the ambient air. The compressed air is then forced into the combustor, where a portion of the air is used to cool the combustor, while the rest is mixed with a fuel and ignited.
Typically, an igniter generates an electrical spark to ignite the air-fuel mixture. The products of the combustion then travel out of the combustor as exhaust through a turbine. The turbine typically has a plurality of rotor airfoils extending from a center body and a plurality of stator airfoils extending to the center body from an engine case surrounding the turbine. As the exhaust expands through the turbine airfoils and stators the turbine and turbine airfoils are forced to rotate around an engine shaft. The turbine and the compressor are connected by a common shaft, which runs through the center of the engine, or in the case of dual spool engines, first and second concentrically mounted shafts. Thus, as the turbine rotates from the exhaust, the associated compressor rotates to bring in and compress new air. Once started, it can thereby be seen that this process is self-sustaining. In industrial gas turbine engines, a power turbine, which rotates freely with respect to the other turbine and the compressor, is rotated by the exhaust to generate an output power.
The turbine must be constructed to endure tremendously high temperatures since the exhaust exiting the combustor is very hot. Therefore, a plurality of cooling holes are typically provided in the turbine to allow a portion of the compressed air from the compressor to flow to the turbine airfoils and act as cooling air to cool the airfoils.
While effective, newer combustor designs generate even higher temperatures in the combustor and therefore produce higher temperature exhaust. Moreover, uneven burning in the combustor, uneven air or fuel spread, or the like may increase the temperature of the already high temperature exhaust to create pockets of very high temperature exhaust leaving the combustor. This increased temperature may occasionally cause heat stress cracks to form on a platform of the turbine airfoils. Additionally, the turbine airfoils cycle between atmospheric and very hot temperatures and back again during every start-up and shut-down of the engine. Over long periods of use, the turbine airfoils may develop heat stress cracks for this reason as well. In such situations, the entire turbine airfoil affected is typically discarded and replaced. As each turbine airfoil is very expensive, this practice results in increased cost to the engine operator.
Therefore, it can be seen that a need exists for a method of repairing turbine airfoils. Specifically, a method of repairing the platform of a turbine airfoil is needed.